1. Field of the Invention
The present invention relates generally to optical imaging systems, and more specifically to systems for imaging non-circular beams of light received from laser diodes into more circular areas of surfaces on of solid-state laser materials or optical fibers.
Laser diodes emit light in beams diverging nonsymmetrically. As FIG. 1T, shows in a top view normal to optical axis OA a typical laser diode 10 PN junction J emitting region has a width W, which coincides with the major axis of elliptical beam B, between 5 and 200 um wide and possibly up to 1 cm wide. Laser diodes 10 over 10 um wide typically emit light that is spatially incoherent across the width of the diode. The emitted light is polarized and diverges typically between 8 and 15 degrees in the "parallel" dimension, i.e., the direction of arrow 11 parallel to width W in FIG. 1T and parallel to plane 12 of PN junction J as seen in side view FIG. 1S.
As shown in FIG. 1S, such a laser diode PN junction J has a height H which is much shorter, typically between 0.5 and 2 um high, in the "perpendicular" dimension, i.e., the dimension perpendicular to PN junction J plane 12. In the perpendicular dimension, laser diode 10 is a diffraction-limited source of spatially coherent light diverging typically 2 to 6 times more rapidly than in the parallel dimension. At half-maximum of light intensity, full angles are in a range between 20 and 50 degrees in the perpendicular dimension. FIG. 1E is an end-view of the beam seen in areas of sections across the FIG. 1T and 1A OA at corresponding lengths. Hence, in the faster diverging perpendicular dimension of light from diode junction J, the intially shorter height H overtakes width W and makes the beam more circular at a distance R. Afterwards, height H becomes the major axis of vertically semi-elliptical beam B.
A laser diode pumped laser, to operate efficiently in the fundamental transverse spatial mode (TEM.sub.00), needs to have a circularly cylindrical TEM.sub.00 mode volume diameter large enough to encompass the beam of light pumped from a laser diode 10 as shown in FIG. 2S. If, for example contrary to FIGS. 2, diode 10 pumps some of its light into regions of gain material outlying the circular TEM.sub.00 mode volume, then, while the laser is below the threshold for lasing in higher order modes, the energy pumped into those outlying regions is lost through fluorescence and heat without practical benefit.
It is possible for semi-elliptical beams of light to be re-shaped into rounder beams. However, it is also important that laser diode 10 pump beam light energy be focused into a circularly cylindrical volume with the smallest diameter attainable, in order to be most efficiently absorbed and converted into power in a solid-state laser. Minimizing a laser's pumped gain region 25 diameter maximizes its gain. Increased gain permits increased output-coupling, which in turn more efficiently dominates fixed losses of the cavity, and also permits approximately inversely proportionately shorter pulses.
For any particular optical beam from a given source has an invariant product of the near-field spot size (diameter) times the sine of the far-field divergence angle times the refractive index of the media through which the beam is passing. Beams B, as shown in FIGS. 2E and 2S, are focused to "waists" Ws, Wt for diode-pumping solid-state lasers. If the beams are focused to diameters which are smaller than optimal then the beams diverge too widely before the beam is absorbed. The length over which all but 1/e of the pump light energy in the beam is absorbed is termed "the absorption length." Consequently, over-focusing, undesirably increases the diameter of the pump beam along its absorption length. Conversely, weaker focusing for reduced divergence is accompanied by increased waist diameters. Optical beams can practically only avoid excessive divergences through being magnified to larger waist diameters. Hence, to optimize end-pumped laser gain and performance, laser diode pump beam diameters are ultimately traded-off against their divergences, in a compromise which images pumping light beams into non-circularly cylindrical volumes of laser materials.
To maximize laser gain, diode pump beam diameter-multiplied-by-sine (divergence) products should be minimized in both the parallel and perpendicular dimensions. Generally, the sizes of diode pump beam B images should be magnified to reduce their far-field divergences in the perpendicular dimension, while being less magnified or demagnified, to reduce their diameters in the parallel dimension. In practice, diode pump beam-reshaping is usually emphasized in the less-easily focused parallel dimension, in which, although the divergence is less, the near-field diameter is much greater and when multiplied by the sine of the far-field divergence gives a greater product than in the perpendicular dimension.